1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method of the semiconductor device.
In this specification, a semiconductor device means any device that can function by utilizing semiconductor characteristics; a semiconductor display device, a semiconductor circuit, an electronic device are all included in the category of the semiconductor device.
2. Description of the Related Art
A technique by which transistors are formed using a semiconductor thin film formed over a substrate having an insulating surface has been attracting attention. Such transistors are applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). As materials of semiconductor thin films applicable to the transistors, there are silicon-based semiconductor materials and oxide semiconductor materials.
For example, amorphous silicon, microcrystalline silicon, polycrystalline silicon, an In—Ga—Zn—O-based oxide semiconductor, and the like are used as active layers of transistors.
Controlling the threshold voltage (Vth) of a transistor is important in terms of on/off characteristics of the transistor. For example, when the threshold voltage is close to 0 V in a transistor, the voltage at which the transistor is turned on can be low, leading to low power consumption.
Patent Document 1 discloses a semiconductor device including a plurality of transistors each including a backgate electrode, a semiconductor active layer provided in contact with the backgate electrode with a first gate insulating film interposed therebetween, and a gate electrode provided in contact with the semiconductor active layer with a second gate insulating film interposed therebetween; and a threshold voltage control circuit which controls the threshold voltage of the plurality of transistors; and a technique in which the threshold voltage is controlled by application of an arbitrary voltage to the back gate electrode by the threshold voltage control circuit.
For example, in the case of using a silicon-based semiconductor material for an active layer, the threshold voltage can be controlled by introducing a Group 13 element or a Group 15 element into the active layer; however, it is known that the introduction of a Group 13 element or a Group 15 element reduces the crystallinity and causes impurity scattering and the like, leading to degradation of transistor characteristics.
Non-Patent Document 1 discloses a technique in which molybdenum oxide is formed on a surface by plasma oxidation of a Mo gate electrode to increase the work function of the gate electrode, so that the threshold voltage shifts to the positive side. This technique shifts the threshold voltage to the positive side but has problems of low resistance of the molybdenum oxide to a later plasma process and recession of the molybdenum oxide layer.